#include <bits/stdc++.h>
using namespace std;
using ll = long long;
#define sz(x) signed(size(x))

inline void solve() {
        int n, m;
        cin >> n >> m;

        vector<vector<int>> cnt(m, vector<int>(26));
        for (int i = 0; i < n; ++i) {
                string s;
                cin >> s;

                for (int j = 0; j < m; ++j)
                        ++cnt[j][s[j] - 'a'];
        }

        int ans = 0;
        for (int i = 0; i < m; ++i)
                ans += n - *max_element(cnt[i].begin(), cnt[i].end());

        cout << ans << '\n';

        return;
}

signed main() {
        cin.tie(0)->sync_with_stdio(0);
        int t = 1;
        while (t--)
                solve();
        return 0;
}